Image sensor for reading image and image reading apparatus including the image sensor

ABSTRACT

An image sensor including a plurality of photoelectric conversion elements arranged in a main scanning direction, a plurality of switching elements connected to respective ones of the photoelectric conversion elements, individually, and a controller for generating a clock signal to control the switching elements. The photoelectric conversion elements are divided into plural groups, each including a predetermined number (N) of the photoelectric conversion elements. The outputs from the photoelectric conversion elements in one group are read out simultaneously in response to the clock signal generated from the controller, thereby attaining a high speed reading of a document.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image sensor. More specifically, thepresent invention relates to a MOS type image sensor that is used as acontact type image sensor (CIS) for reading an image such as a document,and an image reading apparatus such as a facsimile machine utilizing theimage sensor.

2. Description of the Related Art

Conventionally, two types of image sensors are used with an imagereading apparatus such as a facsimile machine; a CCD type image sensorand a MOS type image sensor. Of those, a MOS type linear image sensor isoften used as a contact type image sensor

FIG. 17 shows a conventional MOS type linear image sensor 100. Thisimage sensor 100 includes a plurality of photoelectric conversionelements (phototransistors or photodiodes) 101 that convert lightreflected by the surface of a document into an electric signal as ananalog image signal, a plurality of switching elements (analog switches)102 for reading out the analog image signal from the photoelectricconversion element 101, and a control unit (a shift register) 103 forcontrolling time-sequentially the switching elements 102. These elements101, 102 and 103 are integrated into a single chip LSI.

However, the above-mentioned conventional image sensor, in which a lotof photoelectric conversion elements are arranged in a main scanningdirection, has following problems to satisfy requirements for highresolution and for high speed reading.

Firstly, analog switches that are capable of high speed switchingoperation are required as the switching elements 102. In addition, ananalog amplifier 104 that can amplify the read image signals from thephotoelectric conversion elements at high speed is required.

Secondly, a shift register that includes as many flip-flop circuits asthe switching element 102 is necessary as the control unit 103.Accordingly, a large area for arranging the flip-flop circuits isnecessary, so that the chip size becomes large.

Thirdly, as a frequency of driving clock increases, it is difficult toeliminate high frequency components of the clock signal. As a result, aproblem of electromagnetic interference (EMI) noise occurs.

Fourthly, the following case will be supposed in which an image sensorhaving a lot of photoelectric conversion elements arranged in the mainscanning direction is required in order to read a high resolution image.If a reading condition is changed so as to satisfy low resolutionreading requirement, it is required to thin-out the image signals or toaverage the levels of the image signals in order to convert theresolution after reading at high resolution. Therefore, in spite of lowresolution reading, time period to read the image is long. Thus, it isdifficult to support a request for placing a higher priority on readingspeed than resolution.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an image sensor that iscapable of reading at high speed without increasing the frequency of theclock signal.

Another object of the present invention is to provide an image sensor inwhich switching elements or an analog amplifier is not forced to work athigh speed.

Still another object of the present invention is to provide an imagereading apparatus that has a compact size by decreasing the number ofthe flip-flop circuits and downsizing the area occupied by the shiftregister.

Still another object of the present invention is to provide an imagereading apparatus that suppresses EMI noise

Still another object of the present invention is to provide an imagereading apparatus that can read a document with low resolution and athigh speed, if necessary.

The present invention provides an image sensor including a plurality ofphotoelectric conversion elements arranged in a main scanning direction.Each of the plurality of photoelectric conversion elements generates ananalog image signal corresponding to an amount of incident lightthereon. The photoelectric conversion elements are divided into pluralgroups, each of the plural groups including a predetermined number (N)of the photoelectric conversion elements. More than one switchingelements are connected to respective ones of the plurality ofphotoelectric conversion elements, individually. A control unit controlsthe plurality of switching elements in response to an external clocksignal to simultaneously output the image signals from the predeterminednumber of photoelectric conversion elements in one of the plural groups.

The present invention provides an image reading apparatus including animage sensor. The image sensor includes a plurality of photoelectricconversion elements arranged in a main scanning direction. Each of theplurality of photoelectric conversion elements generates an analog imagesignal corresponding to an amount of incident light thereon. Thephotoelectric conversion elements are divided into plural groups. Eachof the plural groups includes a predetermined number (N) of thephotoelectric conversion elements. More than one switching elements areconnected to respective ones of the plurality of photoelectricconversion elements, individually. A control unit controls the pluralityof switching elements in response to an external clock signal tosimultaneously output the image signals from the predetermined number ofphotoelectric conversion elements in one of the plural groups. Signaloutput lines are provided with a number equal to N. The predeterminednumber of photoelectric conversion elements in each of the plural groupsare connectable with the respective ones of the signal output linesthrough the switching elements, individually, to output the imagesignals from the predetermined number of photoelectric conversionelements to the signal output lines. The image reading apparatus furtherincludes a multiplexer connected to the signal output lines of the imagesensor for multiplexing the image signals transferred through the signaloutput lines; and an analog-to-digital converter for converting theimage signal that is multiplexed by the multiplexer into a digitalsignal.

The present invention provides an image reading apparatus including animage sensor. The image sensor includes a plurality of photoelectricconversion elements arranged in a main scanning direction. Each of theplurality of photoelectric conversion elements generates an analog imagesignal corresponding to an amount of incident light thereon. Thephotoelectric conversion elements are divided into plural groups, eachof the plural groups including a predetermined number (N) of thephotoelectric conversion elements. More than one switching elements areconnected to respective ones of the plurality of photoelectricconversion elements, individually. A control unit controls the pluralityof switching elements in response to an external clock signal tosimultaneously output the image signals from the predetermined number ofphotoelectric conversion elements in one of the plural groups. Signaloutput lines are provided with a number equal to N. The predeterminednumber of photoelectric conversion elements in each of the plural groupsare connectable with the respective ones of the signal output linesthrough the switching elements, individually, to output the imagesignals from the predetermined number of photoelectric conversionelements to the signal output lines. The image reading apparatus furtherincludes a plurality of analog-to-digital converters connected to therespective ones of the signal output lines, individually, for convertingthe image signals transferred through the signal output lines intodigital signals.

The present invention provides an image reading apparatus including animage sensor. The image sensor includes a plurality of photoelectricconversion elements arranged in a main scanning direction. Each of theplurality of photoelectric conversion elements generates an analog imagesignal corresponding to an amount of incident light thereon. Thephotoelectric conversion elements are divided into plural groups. Eachof the plural groups includes a predetermined number (N) of thephotoelectric conversion elements. More than one switching elements areconnected to respective ones of the plurality of photoelectricconversion elements, individually. A control unit controls the pluralityof switching elements in response to an external clock signal tosimultaneously output the image signals from the predetermined number ofphotoelectric conversion elements in one of the plural groups. Signaloutput lines are provided with a number equal to N. The predeterminednumber of photoelectric conversion elements in each of the plural groupsare connectable with the respective ones of the signal output linesthrough the switching elements, individually, to output the imagesignals from the predetermined number of photoelectric conversionelements to the signal output lines. The image reading apparatus furtherincludes a sample-and-hold circuit connected to the signal output linesof the image sensor for temporarily storing the image signal transferredfrom one of the photoelectric conversion elements though thecorresponding ones of the switching elements and the signal outputlines, a multiplexer for multiplexing the image signal storedtemporarily in the sample-and-hold circuit, and an analog-to-digitalconverter for converting the image signal that is multiplexed by themultiplexer into a digital signal.

The present invention provides an image reading apparatus including animage sensor. The image sensor includes a plurality of photoelectricconversion elements arranged in a main scanning direction. Each of theplurality of photoelectric conversion elements generates an analog imagesignal corresponding to an amount of incident light thereon. Thephotoelectric conversion elements are divided into plural groups, eachof the plural groups including a predetermined number (N) of thephotoelectric conversion elements. A plurality of switching elements areconnected to respective ones of the plurality of photoelectricconversion elements, individually. A control unit controls the pluralityof switching elements in response to an external clock signal tosimultaneously output the image signals from the predetermined number ofphotoelectric conversion elements in one of the plural groups. Signaloutput lines are provides with a number equal to N. The predeterminednumber of photoelectric conversion elements in each of the plural groupsare connectable with the respective ones of the signal output linesthrough the switching elements, individually, to output the imagesignals from the predetermined number of photoelectric conversionelements to the signal output lines. The image reading apparatus furtherincludes a multiplexer connected to the image sensor for multiplexingthe image signal transferred from one of the photoelectric conversionelements through the corresponding one of the output signal lines, and asample-and-hold circuit connected to the image sensor for temporarilystoring the image signal transferred from one of the photoelectricconversion elements though the corresponding ones of the switchingelements and the signal output lines. The multiplexer is configured toconnect with the image sensor so that one output signal line connectsthe image sensor to the multiplexer directly and the other output signallines connect the image sensor to the multiplexer through thesample-and-hold circuit.

The present invention provides an image reading apparatus including animage sensor. The image sensor includes a plurality of photoelectricconversion elements arranged in a main scanning direction. Each of theplurality of photoelectric conversion elements generates an analog imagesignal corresponding to an amount of incident light thereon. Thephotoelectric conversion elements are divided into plural groups, eachof the plural groups including a predetermined number (N) of thephotoelectric conversion elements. A plurality of switching elements areconnected to respective ones of the plurality of photoelectricconversion elements, individually. A control unit controls the pluralityof switching elements in response to an external clock signal tosimultaneously output the image signals from the predetermined number ofphotoelectric conversion elements in one of the plural groups. Signaloutput lines are provided with a number equal to N. The predeterminednumber of photoelectric conversion elements in each of the plural groupsare connectable with the respective ones of the signal output linesthrough the switching elements, individually, to output the imagesignals from the predetermined number of photoelectric conversionelements to the signal output lines, The image reading apparatus furtherincludes an analog front-end IC including; an analog amplifier foramplifying an analog input signal received through one of a plurality ofchannels; a multiplexer for multiplexing the analog input signalamplified by the analog amplifier; and an analog-to-digital converterfor converting the analog input signal of each channel that ismultiplexed by the multiplexer into a digital signal. The signal outputlines function as the plurality of channels of the analog front-end IC.The analog front-end IC is connected to the image sensor so that theanalog front-end IC receives the image signal transferred from each ofthe photoelectric conversion elements through the corresponding one ofthe signal output lines as the analog input signal.

The present invention provides an image reading apparatus including animage sensor. The image sensor includes a plurality of photoelectricconversion elements arranged in a main scanning direction. Each of theplurality of photoelectric conversion elements generates an analog imagesignal corresponding to an amount of incident light thereon. Thephotoelectric conversion elements are divided into plural groups. Eachof the plural groups includes a predetermined number (N) of thephotoelectric conversion elements. A plurality of switching elements areconnected to respective ones of the plurality of photoelectricconversion elements, individually. A control unit controls the pluralityof switching elements in response to an external clock signal tosimultaneously output the image signals from the predetermined number ofphotoelectric conversion elements in one of the plural groups. Thepredetermined number of photoelectric conversion elements in each of theplural groups are connectable with the respective ones of the signaloutput lines through the switching elements, individually, to output theimage signals from the predetermined number of photoelectric conversionelements to the signal output lines. The image reading apparatus furtherincludes a multiplexer having signal input terminals with a number equalto N, and a signal output terminal, the multiplexer being connected tothe image sensor so that the multiplexer receives the image signals fromthe plural photoelectric conversion elements that belong to one of thegroups through the signal input terminals in parallel andsimultaneously. An analog-to-digital converter is connected to thesignal output terminal of the multiplexer for converting the analogimage signal supplied sequentially from the multiplexer into a digitalsignal. Resolution switching unit selects one of a high resolution modein which all the image signals from the photoelectric conversionelements that belong to each of the groups are supplied sequentially tothe analog-to-digital converter and a low resolution mode in which theimage signals are thinned out and then supplied to the analog-to-digitalconverter.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned aspects and other features of the invention areexplained in the following description, taken in connection with theaccompanying drawing figures wherein:

FIG. 1 is a circuit diagram showing an image sensor showing a firstembodiment of the present invention;

FIG. 2 is a diagram showing a part of the circuit structure of the imagesensor in detail;

FIG. 3 is a block diagram showing an image reading apparatus utilizingthe image sensor;

FIG. 4 is a timing chart for explaining operations of the image readingapparatus;

FIG. 5 is a timing chart for explaining operations of the image readingapparatus;

FIG. 6 is a block diagram showing an image reading apparatus accordingto a second embodiment;

FIG. 7 is a timing chart for explaining operations of the image readingapparatus of the second embodiment;

FIG. 8 is a block diagram showing an image reading apparatus of a thirdembodiment; is FIG. 9 is a block diagram showing an image readingapparatus of a fourth embodiment;

FIG. 10 is a timing chart for explaining operations of the image readingapparatus of the fourth embodiment;

FIG. 11 is a block diagram showing an image reading apparatus of a fifthembodiment;

FIG. 12 is a circuit diagram showing an image sensor of a sixthembodiment;

FIG. 13 is a circuit diagram showing an image sensor of a seventhembodiment;

FIG. 14 is a block diagram showing an image sensor of an eighthembodiment;

FIG. 15 is a circuit diagram showing a part of an image sensor ofanother embodiment;

FIG. 16 is a diagram showing an image reading apparatus of anotherembodiment; and

FIG. 17 is a circuit diagram showing a conventional image sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments according to the present inventionwill be described in detail with reference to the drawings.

FIG. 1 shows a MOS type linear image sensor 10 according to a firstembodiment. The image sensor 10 is a contact type image sensor (CIS)that is used for a document reading apparatus such as a facsimilemachine.

As shown in FIG. 1, the image sensor (CIS) 10 includes a plurality ofphototransistors P1-Pn and a plurality of analog switches SW1-SWn, eachof which is connected to the respective ones of the phototransistorsP1-Pn, individually. Each of the phototransistors P1-Pn generates ananalog image signal having a level corresponding to an amount ofincident light thereon which is reflected by a document to be scanned.

The plural phototransistors P1-Pn are arranged on a line extending in amain scanning direction at predetermined intervals to constitute aso-called linear image sensor. It should be noted that the predeterminedinterval may be changed depending on the finest resolution of the imagesensor.

The plural phototransistors P1-Pn are divided into plural groups, eachof which includes a predetermined number (N) of neighboringphototransistors. When plural image signals from the predeterminednumber (N) of the phototransistors are read out simultaneously, theanalog switches that are connected to the corresponding one of thephototransistors are turned on simultaneously.

Namely, when the image sensor has 1728 phototransistors arranged in themain scanning direction, the phototransistors are divided into 576groups each of which includes three (N=3) neighboring phototransistors.Three analog switches (SW1-SW3, SW4-SW6, . . . , or SW1726-SW1728) inone group are configured to be turned on simultaneously, so that imagesignals of three phototransistors (P1-P3, P4-P6, . . . , or P1726-P1728)belonging to each group can be read out simultaneously.

A shift register 11 includes a plurality of flip-flop circuits in thesame way as that of the conventional apparatus so as to control theanalog switches SW1-SWn time-sequentially in response to an externalclock signal. In this embodiment, the shift register 11 includes 576flip-flop circuits, and the number 576 is the same as the total numberof the groups of switches.

The total 576 output terminals of flip-flop circuits constituting theshift register 11 are connected to the groups of switches, respectively.Therefore, three analog switches (SW1-SW3, SW4-SW6, . . . , orSW1726-SW1728) in one group of switch are turned on simultaneously everytime an output of the flip-flop circuits is issued to the correspondingone of the switches.

Thus, the shift register 11 responds to the clock signal CLK that isgiven externally after a start pulse SP is supplied, and control signalsSa1, Sa2, . . . are supplied sequentially from the output terminals ofthe flip-flop circuits to the corresponding switch group. For example,when the control signal Sa1 is issued, three analog switches (SW1-SW3)belonging to the first group are turned on simultaneously. Next, whenthe control signal Sa2 is issued, three analog switches (SW4-SW6)belonging to the second group are turned on simultaneously. Then, threeanalog switches belonging to the next group such as the third group, theforth group, etc. are turned on at the same time. Finally, when thecontrol signal Sa576 is delivered, three analog switches (SW1726-SW1728)belonging to the 576-th group are turned on at the same time.

Furthermore, the image sensor 10 of this embodiment includes threesignal output lines L1-L3. The number of signal output lines is the sameas the number of the phototransistors P (analog switches SW) that belongto one group of switch. The plural analog switches SW that are connectedto plural phototransistors P belonging to one group of switch areconnected to different signal output lines L1-L3 so that the imagesignals from the plural (three) phototransistors P belonging to onegroup are supplied in parallel to each of the signal output lines L1-L3.For example, the first analog switch (SW1, SW4, . . . , or SW1726) ofeach group is connected to the first signal output line L1. The secondanalog switch (SW2, SW5, . . . , or SW1727) of each group is connectedto the second signal output line L2. The third analog switch (SW3, SW6,. . . , or SW1728) of each group is connected to the third signal outputline L3.

In addition, the plural (three) signal output lines L1-L3 arerespectively provided with analog amplifiers AMP1-AMP3 for amplifyingthe image signals from the phototransistors P1-Pn, which are read outthrough the analog switches SW1-SWn. The read signals as three channeloutput signals CH1-CH3 are extracted externally from the image sensorthrough the three output terminals OUT1-OUT3 that are disposedcorresponding to the signal output lines L1-L3.

FIG. 2 shows a detailed structure of the phototransistor P and theanalog switch SW of this embodiment. The image sensor 10 includesphototransistors P1-Pn of 1728 in total and analog switches SW1-SWn, ashift register 11 consisting of 576 flip-flop circuits, and 3 analogamplifiers AMP1-AMP3, all of which are formed on a single semiconductorsubstrate. In other words, all the phototransistors P1-Pn, the analogswitches SW1-SWn, the shift register 11, and the analog amplifiersAMP1-AMP3 are integrated into a single chip LSI to produce a singleimage sensor 10.

FIG. 3 shows an example of an image reading apparatus utilizing theimage sensor 10 that has the above-mentioned structure. In the structureshown in FIG. 3, analog read signals that are transmitted through thethree output terminals OUT1-OUT3 of the image sensor 10 as three channeloutput signals CH1-CH3 are converted into a serial data by themultiplexer 20, and supplied to an analog-to-digital converter 30sequentially. In the analog-to-digital converter 30, each of the readsignals is converted into a digital signal.

A control circuit 60 that collectively controls the image sensor 10 andthe image reading apparatus is made of an application-specificintegrated circuit (ASIC). In this embodiment, the control circuit 60includes a CIS control section 61, an analog front-end control section62, and a memory control section 63.

The CIS control section 61 is provided for supplying a start pulse SP, aclock signal CLK to the shift register 11 of the image sensor 10 underthe collective control by a CPU.

The analog front-end control section 62 is provided for supplyingselecting signals SEL1 and SEL2 to the multiplexer 20 and a clock signalfor the analog-to-digital conversion, i.e., a clock signal A/D-CLK fordetermining analog-to-digital conversion timing to the analog front-endcircuit that is made of the multiplexer 20 and the analog-to-digitalconverter 30.

In response to the selecting signals SEL1 and SEL2 for the multiplexer,selection is made on which signal among the three channel output signalsCH1-CH3 that are extracted from the image sensor 10 is to be supplied tothe analog-to-digital converter 30. In addition, the analog front-endcontrol section 62 has a function of selecting resolution of a documentto be read. The resolution is selected from either a “high resolutionmode” or a “low resolution mode” by an external apparatus or anoperation panel. Responding to the selected resolution mode, the analogfront-end control section 62 generates the selecting signals SEL1 andSEL2. If no resolution is designated, the analog front-end controlsection 62 is normally set to read a document with high resolution mode.

The memory control section 63 samples the digital signal that isgenerated in the analog-to-digital converter, 30 and then writes thesampled data in a predetermined area of an image memory (RAM)sequentially.

Furthermore, the clock signal CLK, the selecting signals SEL1 and SEL2,and the clock signal A/D-CLK for the analog-to-digital conversion areissued at timings that are shown in FIG. 4. In addition, each of theclock signal CLK, the selecting signals SEL1 and SEL2, and the clocksignal A/D-CLK for the analog-to-digital conversion is generated inresponse to system clocks produced in the control circuit 60. It shouldbe noted that the system clock has higher frequency than that of theclock signal CLK, the selecting signals SEL1 and SEL2, or the clocksignal A/D-CLK.

Next, an operation of an image reading apparatus that utilizes the imagesensor 10 having the above-mentioned structure will be described withreference to the timing chart shown in FIG. 4.

First, when the phototransistors P1-Pn receive the reflected light,charges are accumulated in the phototransistors P1-Pn in accordance withthe intensity of the light reflected by the document (an object to beread).

When the control signal Sa1 is issued from the shift register 11 inresponse to the clock signal CLK that is supplied to the image sensor10, three analog switches SW1-SW3 belonging to the first group areturned on at the same time so that the charges accumulated in thephototransistors P1-P3 are sent to the output signal lines L1-L3 as theanalog image read signals. Next, the read image read signals areamplified by the analog amplifiers AMP1-AMP3, and then extracted asthree channel output signals CH1-CH3 from the output terminalsOUT1-OUT3.

After that, when the channel output signals CH1-CH3 reach apredetermined level and maintain the level for a predetermined period,signals for selecting a first channel output signal CH1, a certaincombination of SEL1 and SEL2 levels (both SEL1 and SEL2 have a “High”level), are supplied to the multiplexer 20. Then, the first channeloutput signal CH1, i.e., a read signal from the phototransistor P1 (animage signal of a first pixel) is relayed to the analog-to-digitalconverter 30. Next, the first channel output signal CH1 as a digitalsignal D1 including a predetermined bit number of digital code isgenerated in the analog-to-digital converter 30 in synchronization withthe clock signal A/D-CLK for the analog-to-digital conversion.Furthermore, the signal for selecting the first channel output signalCH1 is a signal that connects the first signal input terminal IN1 to thesignal output terminal OUT.

After that, when signals for selecting the second channel output signalCH2, another combination of signals SEL1 and SEL2 (SEL1 has a “High”level, and SEL2 has a “Low” level) are supplied to the multiplexer 20,the second channel output signal CH2, i.e., a read signal from thephototransistor P2 (an image signal of a second pixel) is supplied tothe analog-to-digital converter 30. Next, a second channel output signalCH2 as a digital signal D2 including a predetermined bit number ofdigital code is generated in the analog-to-digital converter 30 insynchronization with the clock signal A/D-CLK for the analog-to-digitalconversion. The signal for selecting the second channel output signalCH2 is a signal that connects the second signal input terminal IN2 tothe signal output terminal OUT.

After that, when signals for selecting a third channel output signalCH3, the other combination of the selecting signals SEL1 and SEL2 (SEL1has a “Low” level, and SEL2 has a “High” level) are supplied to themultiplexer 20, the third channel output signal CH3, i.e., a read signalfrom the phototransistor P3 (an image signal of a third pixel) issupplied to the analog-to-digital converter 30. Next, the read signalfrom the phototransistor P3 as a digital signal D3 including apredetermined bit number of digital code is generated in theanalog-to-digital converter 30 in synchronization with the clock signalA/D-CLK for the analog-to-digital conversion. The signal for selectingthe third channel output signal CH3 is a signal that connects the thirdsignal input terminal IN3 to the signal output terminal OUT.

Next, when the control signal Sa2 is issued from the shift register 11in response to the clock signal CLK supplied to the image sensor 10,three analog switches SW4-SW6 that belong to the second group are turnedon at the same time so that the charges accumulated in thephototransistors P4-P6 are read out as analog image signals on theoutput signal lines L1-L3. These read image signals are amplified by theanalog amplifiers AMP1-AMP3, and then extracted as three channel outputsignals CH1-CH3 from the output terminals OUT1-OUT3.

After that, when the channel output signals CH1-CH3 reach thepredetermined level and maintain the level for the predetermined period,the signals for selecting the first channel output signal CH1, thecertain combination of the selecting signals SEL1 and SEL2 (both SEL1and SEL2 have a “High” level) are supplied to the multiplexer 20. Then,the first channel output signal CH1, i.e., a read signal from thephototransistor P4 (an image signal of a fourth pixel) is relayed to theanalog-to-digital converter 30. This read signal is converted in theanalog-to-digital converter 30 into a digital signal D4 including apredetermined bit number of digital code in synchronization with theclock signal A/D-CLK for the analog-to-digital conversion. Furthermore,the signal for selecting the first channel output signal CH1 is a signalthat connects the first signal input terminal IN1 to the signal outputterminal OUT.

After that, when the signals for selecting the second channel outputsignal CH2, the second combination of the selecting signals SEL1 andSEL2 (SEL1 has a “High” level, and SEL2 has a “Low” level) are suppliedto the multiplexer 20, the second channel output signal CR2, i.e., aread signal from the phototransistor P5 (an image signal of a fifthpixel) is supplied to the analog-to-digital converter 30. Next, thisread signal is converted in the analog-to-digital converter 30 into adigital signal D5 including a predetermined bit number of digital codein synchronization with the clock signal A/D-CLK for theanalog-to-digital conversion. The signal for selecting the secondchannel output signal CH2 is a signal that connects the second signalinput terminal IN2 to the signal output terminal OUT.

Next, when the signals for selecting a third channel output signal CH3,the other combination of the selecting signals SEL1 and SEL2 (SEL1 has a“L” level, and SEL2 has a “H” level) are supplied to the multiplexer 20,the third channel output signal CH3, i.e., a read signal from thephototransistor P6 (an image signal of a sixth pixel) is supplied to theanalog-to-digital converter 30. Next, this read signal is converted inthe analog-to-digital converter 30 into a digital signal D6 including apredetermined bit number of digital code in synchronization with theclock signal A/D-CLK for the analog-to-digital conversion. The signalfor selecting the third channel output signal CH3 is a signal thatconnects the third signal input terminal IN3 to the signal outputterminal OUT.

After that, in the same way, every time when the clock signal CLK issupplied to the image sensor 10, the shift register 11 shifts thecontrol signal Sa contained therein so that the output destination ofthe control signal Sa is shifted in turn. The three analog switches SWthat belong to the group to which the control signal Sa is supplied areturned on simultaneously in response to the control signal Sa. Then, theimage signals of three pixels are read out in parallel from thephototransistors P in the group, and extracted through the multiplexer20 and the analog-to-digital converter 30, as a digital signal Dincluding a predetermined bit number of digital code.

Therefore, when the digital signal D that includes a predetermined bitnumber of digital code and is generated sequentially in theanalog-to-digital converter 30 is stored in a predetermined area of theimage memory (RAM) through a memory write control section 63, the readimage data can be stored in the order of the arrangement in the mainscanning direction.

According to the embodiment described above, the frequency of the clocksignal CLK can be decreased to ⅓, compared with that of the conventionalimage sensor. In addition, it is not required to use a switch thatperforms high speed switching operation as the analog switch SW.Furthermore, the analog amplifier AMP can be used with lower speedoperation. In addition, since the number of the flip-flop circuits inthe shift register 11 can be reduced to ⅓, the entire sensor can bedownsized. Furthermore, since it is possible to read at high speed bythe low speed clock signal CLK, generation of EMI noises can besuppressed.

It should be noted that the number (N) of the photoelectric conversionelements included in one group which are simultaneously turned on andoff can be changeable. Therefore, the frequency of the clock signal CLKcan be reduced depending on the number of the photoelectric conversionelements included in one group. Similarly, any switch with high speedswitching operation is not required. And an analog amplifier with higherspeed operation is not required.

In this way, in order to support the desire for high resolutionscanning, high speed reading operation can be performed in an imagesensor including a plurality of phototransistors arranged in the mainscanning direction by preparing the multiplexer 20 and theanalog-to-digital converter 30 that is suitable for high speedoperation, for example. Furthermore, a photodiode can be used instead ofthe phototransistor.

Next, an operation of reading a document by the image reading apparatusincluding the image sensor 10 described above with the differentresolutions, will be described with reference to FIG. 5.

In the multiplexer 20, the selecting signals SEL1 and SEL2 supplied fromthe analog front-end control section 62 have a constant level withoutinvolving time sequential switching.

When a “high resolution mode” is selected in which high resolutionreading is performed by converting the pixel signal (the image readsignal) from the image sensor 10 into a digital signal, the analogfront-end control section 62 sends a selecting signal to the multiplexer20 for switching the first through the third signal input terminalsIN1-IN3 time-sequentially. On the other hand, when a “low resolutionmode” is selected in which low resolution reading is performed by anexternal apparatus or an operation panel, the analog front-end controlsection 62 sends to the multiplexer 20 the selecting signals withoutinvolving time sequential switching, i.e., selecting signals for fixedlyconnecting the first signal input terminal IN1 to the signal outputterminal OUT (selecting signals in which both SEL1 and SEL2 are fixed to“H” level).

Therefore, in the “low resolution mode”, only the first channel outputsignal CH1 among three channel output signals CH1-CH3 is supplied to theanalog-to-digital converter 30 continuously via the multiplexer 20.Accordingly, when the control signal Sa1 is issued from the shiftregister 11, the read signal from the phototransistor P1 (the firstpixel signal) is extracted as the digital signal as shown in FIG. 5.Next, when the control signal Sa2 is issued, a read signal from thephototransistor P4 (the fourth pixel signal) is extracted as the digitalsignal. After that, in the same way, every time when the control signalSa is shifted sequentially, the seventh, the tenth, . . . , and 1726-thpixel signals are delivered as the digital signal.

Thus, in the “low resolution mode”, the number of effective pixels ofwhich read image signal is extracted is reduced to ⅓. For example,supposing that the reading resolution is “600 dpi” in the “highresolution model”, reading in the “low resolution mode” is performed at“200 dpi”, which is ⅓ of the reading resolution in the “high resolutionmode”.

In addition, only one analog-to-digital conversion operation isperformed every time the control signal Sa is shifted in this “lowresolution mode”. Therefore, compared with the “high resolution mode”that requires three analog-to-digital conversion operations, thefrequency of the clock signal CLK can be increased so that high speedreading can be performed.

Accordingly, reading in the low resolution mode is convenient for thecase where it is necessary to place higher priority on reading speedthan reading resolution.

Though the multiplexer 20 is not switched time-sequentially in theabove-mentioned “low resolution mode”, it is possible to perform thetime sequential switching in accordance with a type of the resolutioneven when the “low resolution mode” is selected.

For example, when reading at “400 dpi” resolution is selected whilereading resolution in the “high resolution mode” is “600 dpi”, any twosignals among the three channel output signals CH1-CH3 may be suppliedto the analog-to-digital converter 30 time sequentially so that readingsignal having “400 dpi” resolution can be obtained.

In this case, the analog-to-digital conversion operation is performedtwice every time when the control signal Sa is shifted. Therefore,compared with the “high resolution model” that requires threeanalog-to-digital conversion operations, the frequency of the clocksignal CLK can be increased so that high speed reading can be performed.

Furthermore, in the above-mentioned embodiment, a lot ofphototransistors P arranged along the main scanning direction of theimage sensor 10 are divided into plural groups each including threeneighboring phototransistors, so that “600 dpi” resolution is obtainedin the “high resolution model”. In another embodiment, if a plurality ofphototransistors P arranged along the main scanning direction aredivided into plural groups each including four neighboringphototransistors by changing an interval between neighboringphototransistors, a read signal having a resolution of “600 dpi”, “450dpi”, “300 dpi” or “150 dpi” can be obtained.

Namely, a multiplexer that has four signal input terminals IN1-IN4corresponding to four channel signal outputs CH1-CH4 is provided. Whenall the signal input terminals IN1-IN4 of the multiplexer are switchedtime-sequentially to send the output signals of the photoelectricconversion elements P1-Pn to the analog-to-digital converter, a readsignal having a high resolution of more than “600 dpi” can be generated.In addition, one of four signal input terminals IN1-IN4 is connectedfixedly to the output terminal OUT without involving time sequentialswitching so that only one channel output signal is supplied to theanalog-to-digital converter. Thus, a read signal having the resolutionof “150 dpi” can be generated.

In addition, any two of four signal input terminals IN1-IN4 can beswitched time sequentially so that any two channel output signals can besupplied sequentially to the analog-to-digital converter 30. Thus, areading signal having “300 dpi” resolution can be delivered.Alternatively, any three of four signal input terminals IN1-IN4 can beswitched time sequentially so that any three channel output signals canbe supplied sequentially to the analog-to-digital converter 30. Thus, areading signal having “450 dpi” resolution can be delivered.

In the same way, when the phototransistors P of the image sensor 10 aredivided into groups each including six phototransistors by changing aninterval between neighboring phototransistors P arranged in the mainscanning direction, a read signal having the resolution of “600 dpi”,“500 dpi”, “400 dpi”, “300 dpi”, “200 dpi” or “100 dpi” can be obtained.

If the number of the phototransistors P that belong to one group isincreased, a high speed analog-to-digital converter may be required.However, when the same number of signal output lines as that ofphototransistors P in one group are divided into several groups, and ananalog-to-digital converter is provided for each group of the signaloutput lines, high speed reading operation can b performed as a wholeeven if the analog-to-digital converter operates at a relatively lowspeed.

FIGS. 6 and 7 show an image reading apparatus according to a secondembodiment of the present invention. The image reading apparatus shownin FIG. 6 includes a sample-and-hold circuit 50 for storing temporarilythree channel output signals CH1-CH3, in addition to the elements of theimage reading apparatus shown in FIG. 1, so that a signal that istemporarily stored by the sample-and-hold circuit 50 is supplied to themultiplexer 20. Namely, three channel output signals CH1-CH3 of theimage sensor 10 are not supplied directly from the photoelectricconversion elements to the multiplexer 20.

In other words, as shown in FIG. 7, even if it takes a certain time forstabilizing the three channel output signals CH1-CH3 of the image sensor10 to be a predetermined output level, the sample-and-hold circuit 50stores temporarily the level of three channel output signals CH1-CH3 inresponse to a clock signal S/H-CLK for the sample-and-holding suppliedfrom the analog front-end control section 62 when the level of thechannel output signal reaches a predetermined level and maintains thelevel for a certain time period. It should be noted that the clocksignal S/H-CLK is issued from the analog front-end control section 62when the channel output signal reaches the second maximum level lowerthan the maximum level.

When a signal for selecting the first channel output signal CH1 issupplied to the multiplexer 20 in the above-mentioned state so that thefirst channel output signal CH1 is selected, the first channel outputsignal CH1 that is temporarily stored by the sample-and-hold circuit 50is transmitted to the analog-to-digital converter 30, and then convertedfrom analog to digital in the analog-to-digital converter 30. In orderto select the first channel output signal CH1, both SEL1 and SEL2 have a“High” level.

Next, when a signal for selecting the second channel output signal CH2is supplied to the multiplexer 20 so that the second channel outputsignal CH2 is selected, the second channel output signal CH2 that istemporarily stored by the sample-and-hold circuit 50 is sent to theanalog-to-digital converter 30, and then converted from analog todigital by the analog-to-digital converter 30. Concerning the signal forselecting the second channel output signal CH2, SEL1 has a “High” levelwhile SEL2 has a “Low”s level.

Furthermore, when a signal for selecting the third channel output signalCH3 is supplied to the multiplexer 20 so that the third channel outputsignal CH3 is selected, the third channel output signal CH3 that istemporarily stored by the sample-and-hold circuit 50 is sent to theanalog-to-digital converter 30 through the multiplexer 20, and thenconverted from analog to digital by the analog-to-digital converter 30.Concerning the signal for selecting the third channel output signal CH3,SEL1 has a “Low” level while SEL2 has a “High” level.

Therefore, in the second embodiment, the channel output signals CH1-CH3may be converted from analog to digital by utilizing a period duringwhich the clock signal S/H-CLK for the sample-and-holding is supplied,i.e., a period of clock signal CLK that is supplied to the shiftregister 11. Therefore, the multiplexer 20 and the analog-to-digitalconverter 30 are not required to operate at higher speed as themultiplexer 20 and the analog-to-digital converter 30 in the firstembodiment described above.

The sample-and-hold circuit 50 in the second embodiment does notnecessarily store all the three channel output signals CH1-CH3temporarily. For example, as the third embodiment shown in FIG. 8, asample-and-hold circuit 50 a for temporarily storing the second channeloutput signal CH2 and the third channel output signal CH3 may beprovided so that the same effect can be obtained as in the secondembodiment.

Also in the second and the third embodiments, “low resolution mode”reading can be performed in the same way as in the first embodimentdescribed above, by sending to the multiplexer 20 the selecting signalsthat do not involve time sequential switching, i.e., the selectingsignals for fixedly connecting the first signal input terminal IN1 tothe signal output terminal OUT (both the selecting signals SEL1 and SEL2have a “Low” level). Particularly, in the third embodiment shown in FIG.8, the function of the sample-and-hold circuit 50 a can be stopped inthe case where the “low resolution mode” is selected.

FIG. 9 shows an image reading apparatus of a fourth embodiment. Eitherone of the “low resolution mode (hereinafter referred to as a first lowresolution mode)” that can be selected in the first through the thirdembodiments described above and a “second low resolution mode” obtainedby averaging a predetermined number of pixels to obtain one pixel havingthe averaged signal level can be selected depending on an application.

The image reading apparatus of a fourth embodiment, a second multiplexer40 having three inputs and one output is provided in addition to themultiplexer 20 having the same structure as the above-described firstembodiment.

The second multiplexer 40 is provided for averaging the three channeloutput signals CH1-CH3 that are generated from the image sensor 10. Thesecond multiplexer 40 is structured so that the channel output signalsCH1-CH3 are supplied to three signal input terminals IN1-IN3 of themultiplexer 40 through resisters 70 that constitute a part of anaveraging circuit. Each of the resisters 70 reduces the level of thechannel output signal by a predetermined ratio for averaging the channeloutput signals CH1-CH3. The resistors 70 have an electrical resistancedepending on the number of channel output signals to be averaged. Andthe averaged signal generated in the multiplexer 20 is supplied to theanalog-to-digital converter 30.

Namely, the second multiplexer 40 can have five states as follows inresponse to three selecting signals SEL3-SEL5 that are supplied throughthe analog front-end control section 62. One is a first selection statewhere none of the three signal input terminals IN1-IN3 is connected tothe output terminal OUT. Another is a second selection state where allthe three signal input terminals IN1-IN3 are connected to the outputterminal OUT. In third through fifth selection states, any two of threesignal input terminals IN1-IN3 (the first and the second signal inputterminals IN1 and IN2, or the first and the third signal input terminalsIN1 and IN3, or the second and the third signal input terminals IN2 andIN3) are connected to the output terminal OUT, respectively. Especiallyin the second selection state, the second multiplexer 40 operates as anaveraging circuit that cooperates with the resisters 70 for averagingthe three channel output signals CH1-CH3 and supplying the averagedsignal to the analog-to-digital converter 30.

Furthermore, the second multiplexer 40 is structured in the same way asthe multiplexer 20 and includes a plurality of analog switches. However,it should be noted that the second multiplexer 40 is structured not toperform the inherent operation of the multiplexer, i.e., turning on theplural analog switches sequentially.

In the fourth embodiment, if the “high resolution mode” or the “firstlow resolution mode” is selected by an external apparatus, an operationpanel, the second multiplexer 40 is first switched to the firstselection state (the state where none of the three signal inputterminals IN1-IN3 is connected to the output terminal OUT), and then thethree signal input terminals IN1-IN3 of the multiplexer 20 are switchedtime-sequentially in the same way as in the first embodiment describedabove. Thus, the multiplexer 20 can supply the three channel outputsignals CH1-CH3 sequentially to the analog-to-digital converter 30, sothat high resolution reading can be performed. In addition, when thefirst signal input terminal IN1 of the multiplexer 20 is fixedlyconnected to the output terminal OUT, only the first channel outputsignal CH1 is supplied to the analog-to-digital converter 30, so thatthe low resolution reading with so-called thinning-out can be performedby using only one out of every three image signals.

In addition, while the multiplexer 20 is maintained in the state wherenone of the three signal input terminals IN1-IN3 is connected to theoutput terminal OUT in accordance with the selecting signals SEL1 andSEL2 that are supplied from the analog front-end control section 62 (thestate where both SEL1 and SEL2 have a “L” level), the second multiplexer40 can be switched to the second selection state (the state where allthe three signal input terminals IN1-IN3 are connected to the outputterminal OUT). In this case, the second multiplexer 40 can supply theaveraged signal of the three channel output signals CH1-CH3 to theanalog-to-digital converter 30. Therefore, as shown in FIG. 10, forexample, at the time point when the control signal Sa1 is issued fromthe shift register 11, the averaged signal of the read signals from thephototransistors P1-P3 (the first through the third pixel signals) isgenerated as the digital signal corresponding to the first group. At thetime point when the control signal Sa2 is issued, the averaged signal ofthe read signals from the phototransistors P4-P6 (the fourth throughsixth pixel signals) is generated as the digital signal corresponding tothe second group. In the same way, every time the output destination ofthe control signal Sa is sequentially shifted among the groups of thephototransistors P, the averaged signal of th seventh through ninth,tenth through twelfth, . . . , or 1726-th through 1728-th pixel signalsis generated as the digital signal corresponding to each of the groups.

Thus, also in the “second low resolution mode”, in the same way as inthe “first low resolution mode”, the number of the signals that aregenerated as the read image signals can be reduced to ⅓. Accordingly, ifthe reading resolution in the “high resolution mode” is “600 dpi” forexample, reading is performed at the resolution of “200 dpi”.

In addition, also in the “second low resolution mode”, analog-to-digitalconversion operation may be performed once every time the outputdestination of the control signal Sa is shifted among the groups ofphototransistors P. Therefore, compared with the “high resolution mode”that requires three analog-to-digital conversion operations for onecontrol signal Sa, the frequency of the clock signal CLK can beincreased so that high speed reading can be performed.

Accordingly, reading in the low resolution mode is advantageous for thecase where high speed reading is required while ensuring to maintaincontinuity of thin lines.

Furthermore, the second multiplexer 40 can switch among the thirdthrough the fifth selection states, so as to average any two channelsignals among the three channel output signals CH1-CH3 and to supply theaveraged signal to the analog-to-digital converter 30. Therefore,various image signals can be generated as the read image signal in the“low resolution mode”.

FIG. 11 shows an image reading apparatus of a fifth embodiment accordingto the present invention. This image reading apparatus includes an IChaving all analog and hybrid signal functions necessary for digitizingoutputs of various types of MOS image sensors or CCD image sensors suchas a correlation double sampling (CDS) function, a gain/offsetcorrection function, and an analog-to-digital conversion function.Namely, this image reading apparatus is produced with an analogfront-end IC (integrated circuit) for an image sensor processing system.

The analog front-end IC 40 shown in FIG. 11 has the same structure as acommercial analog front-end IC. Namely, the analog front-end IC 40includes correlation double sampling circuits 41 a-41 c, offsetadjustment circuits 42 a-42 c, programmable gain amplifiers 43 a-43 c, amultiplexer 44, an analog-to-digital converter 45, an interface 46 and aregister portion 47.

In general, this type of analog front-end IC 40 includes three channelsso as to support a color image sensor that generates red, green and blueoutput signals. Therefore, the analog front-end IC 40 includes thecorrelation double sampling circuits 41 a-41 c described above, theoffset adjustment circuits 42 a-42 c and the programmable gainamplifiers 43 a-43 c for each channel.

The correlation double sampling circuits 41 a-41 c are provided foreliminating components which may cause noises and errors from pixelsignals supplied from the CCD type image sensor. The correlation doublesampling circuits 41 a-41 c, as known well, performs sampling of pixelsignals received from the CCD type image sensor twice at two differenttimings to eliminate error voltage that may be caused by electriccharges when the clock of the shift register (CCD) changes from the “L”level to the “H” level. However, when used with the MOS type imagesensor 10 of the present invention, the correlation double samplingcircuits 41 a-41 c are set not to have the correlation double samplingfunction thereof (CDS off mode setting). In this embodiment, thecorrelation double sampling circuits 41 a-41 c operate assample-and-hold circuits for storing the pixel signals received from theMOS type image sensor 10 temporarily for each channel.

Each of the offset adjustment circuits 42 a-42 c includes adigital-to-analog converter DAC and an adder ADD for each channel, sothat an offset voltage is added to an input signal of each channel.Namely, the register portion 47 is provided with an offset registerOFF-REG, which memorizes a set value of the offset indicating offsetvoltage that is applied by the offset adjustment circuits 42 a-42 c foreach channel. Each of the offset adjustment circuits 42 a-42 c for eachchannel adds first the offset voltage corresponding to the set value ofthe offset thereof that is memorized in the offset register OFF-REG tothe image signal that is transmitted from the correlation doublesampling circuits 41 a-41 c. Next, each of the offset adjustmentcircuits 42 a-42 c supplies the image signal, to which the offsetvoltage is added, to the corresponding channel of the programmable gainamplifiers 43 a-43 c.

The programmable gain amplifiers 43 a-43 c are well-known analogamplifiers that can adjust a gain to the input signal. Each of theprogrammable gain amplifiers 43 a-43 c amplifies the image signal of thecorresponding channel by a gain corresponding to the gain set value foreach channel that is memorized in a gain register GAIN-REG of theregister portion 47. The amplified image signal is then supplied to themultiplexer 44.

The multiplexer 44 and the analog-to-digital converter 45 operate in thesame way as the multiplexer 20 and the analog-to-digital converter 30 inthe first through the third embodiments described above. The multiplexer44 selects one of the input image signals from the three programmablegain amplifiers 43 a-43 c to output the selected signal. The multiplexer44 supplies the image signal that is amplified by each of theprogrammable gain amplifiers 43 a-43 c sequentially to theanalog-to-digital converter 45. The analog-to-digital converter 45converts the image signal received from the multiplexer 44 into adigital signal having a predetermined bit number of digital code andthen sends the digital signal to outside.

The interface 46 is provided for writing many types of data into theregister portion 47 through the analog front-end control section 62 ofthe control circuit 60. The interface 46 writes an optimal set value ofthe offset and an optimal gain set value that are obtained by acalibration operation in pre-scanning before real scanning into a memoryportion of the register portion 47 corresponding to each channel.

In the apparatus having the above described structure according to thefifth embodiment, a general-purpose analog front-end IC can be used forconverting the three channel read image signals, that are transmittedfrom the signal output lines of the image sensor 10, into a digitalsignal for each pixel signal. Therefore, compared with the first throughthe third embodiments described above, a circuit structure diagram canbe simplified substantially.

In addition, it is possible to utilize the offset adjustment function orthe gain adjustment function of the analog front-end IC to correctvariation among plural channels easily and appropriately. Furthermore,since a special circuit for the sample-and-holding is not necessary,wiring for the circuit connection is not required, thereby reducingexternal noises.

The structure in the above embodiment is not limited to the illustratedone. The image reading apparatus of present invention may have astructure shown in FIG. 12 as a sixth embodiment. The image sensor 10includes a multiplexer circuit 20 a having the same function as themultiplexer 20 described above. In addition, the image reading apparatusmay include a sample-and-hold circuit (not illustrated) having the samefunction as the sample-and-hold circuits 50 and 50 a.

If the image sensor 10 includes the multiplexer circuit 20 a, an analogamplifier AMP may be provided subsequent to the multiplexer circuit 20 aas shown in FIG. 13 as a seventh embodiment. This image readingapparatus may be advantageous in terms of cost.

Furthermore, in the above-described embodiment, three channel signaloutputs are multiplexed by the multiplexer 20 so as to be converted intoa digital signal by a single analog-to-digital converter 30. However, asshown in FIG. 14 as an eighth embodiment, it is possible to providethree analog-to-digital converters 30 a-30 c for each channelcorresponding to three channel output signals CH1-CH3.

In this structure, even if high speed reading operation is required, avery high speed analog-to-digital converter 30 is not needed. Inaddition, it is also advantageous that a multiplexer 20 that can operateat a high speed is not necessary.

Though some embodiments that concretely describe the present inventionare described above, an image sensor and an image reading apparatususing the image sensor according to the present invention are notlimited to the above-described embodiments but can be embodied indifferent manners. For example, the photoelectric conversion element ofthe image sensor 10 is not limited to the phototransistor shown in FIG.2, but can be a photodiode as an image sensor shown in FIG. 15.Furthermore, any appropriate type of photoelectric conversion elementmay be used.

In addition, if the image sensor 10 is a contact type image sensor(CIS), an image sensor that is very long in the main scanning directionis necessary, as the width of a document to be read becomes large in themain scanning direction. However, there is a limit to manufacture alarge image sensor on a single semiconductor chip.

In order to solve the above problem, a plurality of image sensors 10 maybe aligned along the main scanning direction as an image sensor shown inFIG. 16. In operation thereof, an output of the final flip-flop circuitof the shift register 11 constituting each of the image sensors 10 maybe supplied to the neighboring image sensor 10 as a start pulse SP.

An image sensor according to the present invention includes a pluralityof photoelectric conversion elements that are divided into plural groupseach including a predetermined number (N) of neighboring photoelectricconversion elements, a plurality of switching elements, and a controlunit for controlling the plural switching elements so as to read imagesignals from the plural photoelectric conversion elements belonging toeach of the groups simultaneously for each group in accordance with anexternal clock signal. According to this structure, the frequency of theclock signal can be decreased to 1/N compared with the conventionalimage sensor. Thus, switching elements that are capable of high speedswitching are not required. In addition, since the number of flip-flopcircuits of the shift register included in the control unit can be alsoreduced to 1/N, a dimension of the entire sensor can be reduced. Inaddition, since high speed reading can be performed by a low speed clocksignal, generation of EMI noise can be suppressed.

According to the image sensor of the present invention, the read imagesignals from the plural photoelectric conversion elements are sent tothe corresponding signal output line inside the image sensor. The numberof the signal output lines is the same as the number (N) of thephotoelectric conversion elements belonging to one of the groups.Therefore, the number of terminals for outputting signals from the imagesensor can be minimized so that electric connection to a subsequentmultiplexer or a subsequent analog-to-digital converter can besimplified.

According to the image sensor of the present invention, an analogamplifier for amplifying the image signal is provided to each of thesignal output lines in the image sensor. Therefore, a read image signalfrom each photoelectric conversion element can be extracted from theimage sensor as an analog signal having a sufficient level.

According to the image sensor of the present invention, the imagesignals from the photoelectric conversion elements are multiplexed, sothat image signals from plural (N) photoelectric conversion elements areextracted from the image sensor as a serial data. Therefore, the numberof terminals for outputting signals from the image sensor can beminimized, so that electric connection to a subsequent analog-to-digitalconverter can be simplified.

According to the image sensor of the present invention, the imagesignals from the photoelectric conversion elements are multiplexed, sothat image signals generated from plural (N) photoelectric conversionelements are extracted from the image sensor as a serial data in theimage sensor. Therefore, the number of terminals for outputting signalsfrom the image sensor can be minimized, so that electric connection to asubsequent analog-to-digital converter can be simplified.

According to the image sensor of the present invention, the read imagesignals from the photoelectric conversion elements are storedtemporarily by a sample-and-hold circuit, and then are multiplexed by amultiplexer. After that, the image signals extracted from the imagesensor as a serial data. Accordingly, when the read signal from eachphotoelectric conversion element is converted into a digital signal bythe analog-to-digital converter, a time necessary for the conversionprocess can be sufficiently provided. Therefore, an analog-to-digitalconverter operating at a high speed is not necessary.

According to an image reading apparatus of the present invention, amultiplexer for multiplexing image signals supplied from the signaloutput lines of the image sensor, an analog-to-digital converter forconverting the image signal that is multiplexed by the multiplexer intoa digital signal for each of the image signals supplied from thephotoelectric conversion elements are provided outside the image sensor.Therefore, a plurality of image signals supplied from the photoelectricconversion elements can be converted into digital signals by a singleanalog-to-digital converter.

According to the image reading apparatus of the present invention, eachof the plural image signals supplied from the signal output lines of theimage sensor can be converted into a digital signal independently byeach of the plural analog-to-digital converters. Therefore, even if theanalog-to-digital converter operates at relatively low speed, it ispossible to perform high speed reading operation.

According to the image reading apparatus of the present invention, theplural read image signals that are supplied from the signal output linesare stored temporarily by the sample-and-hold circuit, multiplexed bythe multiplexer and extracted from the image sensor as a serial data.Therefore, when the read signal from the photoelectric conversionelements are converted into digital signals by the analog-to-digitalconverter, a time period required for the converting operation can besecured. Accordingly, an analog-to-digital converter operating at a highspeed is not required.

According to an image reading apparatus of the present invention, theplural read image signals supplied from the signal output lines of theimage sensor can be converted into digital signals for each pixel signalby using the analog front-end IC, so that a structure of circuits can besimplified substantially.

In addition, resolution switching units performs time sequentialswitching of the image signals from the plural photoelectric conversionelements by using the multiplexer. Thus, not only high resolutionreading but also low resolution reading can be performed by supplyingonly a part of the image signals to the analog-to-digital converter.

According to an image reading apparatus of the present invention, onlythe image signal from one of the plural signal input terminals of themultiplexer is supplied to the analog-to-digital converter, so that lowresolution read image signal can be obtained easily and at high speed.

According to an image reading apparatus of the present invention, someof the plural signal input terminals of the multiplexer is switchedtime-sequentially, so that the image signals selected from plural signalinput terminals are supplied to the analog-to-digital converter.Therefore, low resolution that corresponds to, for example, 2/3resolution of that in high resolution mode can be obtained easily.

According to an image reading apparatus of the present invention, someimage signals selected among plural photoelectric conversion elementsbelonging to one of the groups can be averaged and supplied to theanalog-to-digital converter. Therefore, low resolution reading can beperformed easily by averaging the plural image signals, if necessary, inaddition to low resolution reading by so-called simple thinning-oututilizing only the image signals at a constant pitch.

According to an image reading apparatus of the present invention, asignal that is an average of all signals from the photoelectricconversion elements of each group is supplied to the analog-to-digitalconverter, using all the image signals effectively, Therefore, it isvery useful if high speed reading is desired while securing to keep afine line.

It is understood that the foregoing description and accompanyingdrawings set forth the preferred embodiments of the invention at thepresent time. Various modifications, additions and alternative designswill, of course, become apparent to those skilled in the art in light ofthe foregoing teachings without departing from the spirit and scope ofthe disclosed invention. Thus, it should be appreciated that theinvention is not limited to the disclosed embodiments but may bepracticed within the full scope of the appended claims.

1. An image reading apparatus comprising: an image sensor that includes:a plurality of photoelectric conversion elements arranged in a mainscanning direction, each of the plurality of photoelectric conversionelements generating an analog image signal corresponding to an amount ofincident light thereon, the plurality of photoelectric conversionelements being divided into plural groups, each of the plural groupsincluding a predetermined number (N) of the photoelectric conversionelements; a plurality of switching elements connected to respective onesof the plurality of photoelectric conversion elements, individually; acontrol unit that controls the plurality of switching elements inresponse to an external clock signal to simultaneously output the imagesignals from the predetermined number of photoelectric conversionelements in one of the plural groups; and signal output lines with anumber of N, wherein the predetermined number of photoelectricconversion elements in each of the plural groups are connectable withthe respective ones of the signal output lines through the switchingelements, individually, to output the image signals from thepredetermined number of photoelectric conversion elements to the signaloutput lines; a multiplexer connected to the image sensor formultiplexing the image signal transferred from one of the photoelectricconversion elements through the corresponding one of the signal outputlines; and a sample-and-hold circuit connected to the image sensor fortemporarily storing the image signals transferred from the photoelectricconversion elements in one group though the corresponding ones of theswitching elements and the signal output lines; wherein the multiplexeris configured to connect with the image sensor so that one signal outputline connects the image sensor to the multiplexer directly and the othersignal output lines connect the image sensor to the multiplexer throughthe sample-and-hold circuit.
 2. The image reading apparatus according toclaim 1, further comprising an analog-to-digital converter forconverting the image signal that is multiplexed by the multiplexer intoa digital signal.
 3. The image reading apparatus according to claim 1,wherein the multiplexer first receives the image signal transferred fromone of the plural groups through the one signal output line among theimage signals transferred from one of the plural groups.
 4. The imagereading apparatus according to claim 2, further comprising: a resolutionswitching unit that selects one of a high resolution mode in which allthe image signals from the photoelectric conversion elements that belongto each of the groups are supplied sequentially to the analog-to-digitalconverter, and a low resolution mode in which the image signals arethinned out and then supplied to the analog-to-digital converter,wherein the multiplexer receives only the image signal transferredthrough the one signal output line when the low resolution mode isselected.
 5. The image reading apparatus according to claim 1, whereinthe predetermined number (N) is at least
 3. 6. An image readingapparatus comprising: an image sensor that includes: a plurality ofphotoelectric conversion elements arranged in a main scanning direction,each of the plurality of photoelectric conversion elements generating ananalog image signal corresponding to an amount of incident lightthereon, the plurality of photoelectric conversion elements beingdivided into plural groups, each of the plural groups including apredetermined number (N) of the photoelectric conversion elements; aplurality of switching elements connected to respective ones of theplurality of photoelectric conversion elements, individually; and acontrol unit that controls the plurality of switching elements inresponse to an external clock signal to simultaneously output the imagesignals from the predetermined number of photoelectric conversionelements in one of the plural groups, wherein the predetermined numberof photoelectric conversion elements in each of the plural groups areconnectable with the respective ones of the signal output lines throughthe switching elements, individually, to output the image signals fromthe predetermined number of photoelectric conversion elements to thesignal output lines; a multiplexer having signal input terminals with anumber equal to N and a signal output terminal, the multiplexer beingconnected to the image sensor so that the multiplexer receives the imagesignals from the plural photoelectric conversion elements that belong toone of the groups through the signal input terminals simultaneously; ananalog-to-digital converter connected to the signal output terminal ofthe multiplexer for converting the analog image signal suppliedsequentially from the multiplexer into a digital signal; and resolutionswitching unit that select one of a high resolution mode in which allthe image signals from the photoelectric conversion elements that belongto each of the groups are supplied sequentially to the analog-to-digitalconverter and a low resolution mode in which the image signals arethinned out and then supplied to the analog-to-digital converter.
 7. Theimage reading apparatus according to claim 6, wherein the resolutionswitching unit is configured to supply the image signals received fromonly one of the signal input terminals to the analog-to-digitalconverter, when the low resolution mode is selected.
 8. The imagereading apparatus according to claim 6, wherein the resolution switchingunit is configured to select the signal input terminals among all thesignal input terminals to supply the image signal from the selectedsignal input terminals to the analog-to-digital converter by switchingthe selected signal input terminals time-sequentially when the lowresolution mode is selected.
 9. The image reading apparatus according toclaim 6, further comprising an averaging circuit for averaging the imagesignals selected among the image signals received from the pluralphotoelectric conversion elements that belong to one of the groups,wherein the resolution switching unit selects another low resolutionmode in which the averaging circuit is used to lower the resolution ofthe image in addition to the high and low resolution modes, wherein anoutput signal of the averaging circuit is supplied to theanalog-to-digital converter when the another low resolution mode isselected.
 10. The image reading apparatus according to claim 9, whereinthe averaging circuit is configured to average all the signals receivedfrom the photoelectric conversion elements that belong to one of thegroups and to supply the averaged signal to the analog-to-digitalconverter.